1. Field of the Invention
This invention relates to novel N-(4-pyridazinyl)-N'-phenylureas and -phenylthioureas (often referred to as N-(4-pyridazinyl)-N'-phenylureas), agricultural composition comprising such as an active ingredient(s) and methods of treating plants therewith.
2. Description of the Prior Art
In the agricultural and horticultural fields, increasing attention has been directed to plant growth regulators which accelerate or suppress the growth of plants.
In particular, recently techniques for controlling plant growth utilizing the hormone activity of chemicals such as cytokinin or gibberellin have been investigated. Plant growth regulators exhibiting cytokinin-like hormonal activity (hereafter referred to as cytokinin hormone activity) can regulate plant growth when used in vary small amounts. On the other hand, plant growth can sometimes be suppressed when such chemicals are employed in high amounts, i.e., in amounts over those effective to accelerate plant growth (sometimes referred to as overdose use).
Accordingly, while the terms "plant growth regulation" and "plant growth regulator" used herein refer primarily to acceleration, they sometimes refer to the suppression of plant growth in a broad sense. Such seemingly contrary activities by the plant growth regulator are characteristic of cytokinin activity. In this regard, herbicides for which cytokinin activity has clearly been established are not known, though some herbicides have been established to have auxin activity.
Typical plant growth regulators known as having cytokinin hormone activity are 6-benzyladenine, kinetin, and N-(4-pyridyl)-N'-phenylurea. Of these, N-(4-pyridyl)-N'-phenylurea is disclosed in British Pat. No. 1,122,662 as a representative of N-heterocyclic aromatic-N'-aryl ureas which may be substituted by halogen, alkyl or nitro at the heterocyclic aryl moiety. The present inventors have previously found that N-(4-pyridyl)-N'-phenylureas having specific substituents such as halogen atoms are potent plant growth regulators when the substitution occurs at the 2-position of the 4-pyridyl moiety and filed as Ser. No. 947,468 dated Oct. 2, 1978 now U.S. Pat. No. 4,193,788.
Despite the above, the development of plant growth regulators having more improved effects is still desired by the art.